1. Field of the Invention
This invention relates to an improved process for preparing ethylene dichloride (1,2-dichloroethane). More particularly, the invention relates to an improved method whereby substantially pure ethylene dichloride is prepared by the liquid phase reaction between ethylene and chlorine. Ethylene dichloride has many uses, not the least of which is as raw material in the manufacture of vinyl chloride.
Most present-day commercial processes for the production of ethylene dichloride, as well as the modified process of the present invention, are based on the liquid phase reaction between ethylene and chlorine within a body of liquid ethylene dichloride. The reaction between chlorine and ethylene is highly exothermic and the temperature in the reaction medium must be controlled to prevent the formation of substitution reaction products which limit the value of the ethylene dichloride product for certain commercial processes and particularly for its use in the production of vinyl chloride by pyrolysis.
2. Description of the Prior Art
U.S. Pat. No. 2,393,367 discloses a procedure for the preparation of ethylene dichloride wherein ethylene and chlorine are introduced into a water-cooled reactor containing ethylene dichloride with a small amount of ferric chloride as a substitution reaction depressant. The reactor employed is constructed with a plurality of internal tubes which carry a cooling agent to thereby maintain the reaction medium at a temperature of about 40.degree. C., which is well below the boiling point of ethylene dichloride (83.5.degree. C.). As reported, at least about 0.5% of the chlorine added reacts by substitution to form undesired higher chlorinated products. The product provided by this process would therefore require various purification steps before achieving suitability as a raw material for the production of vinyl chloride, as well as for many other uses to which it might be put. The primary contribution of this expired patent to the art is the use of ferric chloride to depress the unwanted substitution reactions and thereby reduce the degree of contamination of the final product with undesirable highly chlorinated substitution products.
U.S. Pat. No. 2,929,852 also discloses a modified process directed to the liquid phase reaction between ethylene and chlorine. In this disclosure, ethylene dichloride is not only the product but serves as the reaction medium as well, and direct temperature control is achieved by utilizing the exothermic heat of reaction to vaporize that portion of the ethylene dichloride which becomes the purified product. The temperature of the reaction medium is held between 80.degree. and 120.degree. C. at pressures between 0 and 25 lbs. per square inch gauge (psig). The vaporized portion of the ethylene dichloride reaction medium is conducted to a rectification column. Ethylene dichloride is separated from the higher chlorinated reaction products and the ethylene dichloride product is removed in liquid phase from the rectification zone. The product of this process, although of greater purity than that obtainable from the process of U.S. Pat. No. 2,393,367 previously cited, nevertheless is less pure than the product of the present invention.
We have now found surprisingly, in contrast to the teachings of the prior art, that the method of controlling the exothermic reaction by circulating the reaction medium through an external heat exchanger, in combination with the method of removing heat from the reaction medium by the continuous vaporization of a portion of said reaction medium to thus maintain the temperature from 83.degree. C. to higher temperatures depending on the quantity of high boilers present, preferably between 83.degree. to 90.degree. C., has an appreciable and beneficial effect on the degree of purity of the final ethylene dichloride product. We have found the method of the present invention to be superior in this respect, to the methods of control involving internal coolers, or cooling exclusively, by utilization of the heat of vaporization of a volatile reaction medium.
In the production of vinyl chloride by pyrolysis, not only is the assay of the ethylene dichloride raw material important, but the nature of its impurities as well. Certain chlorinated contaminants such as ethyl chloride and 1,1,2-trichloroethane are especially detrimental when present, even in minute quantities. For this reason, it has heretofore generally been necessary, regardless of the manner of preparation of the ethylene dichloride, to subject it to costly purification steps before it could be pyrolyzed for the production of vinyl chloride. The product of the process of the present invention, however, is essentially free of chlorinated substitution reaction products including ethyl chloride, 1,1,2-trichloroethane and other undesirable contaminants and therefore said process may be employed in the production of high quality vinyl chloride by pyrolysis, without further purification.